1. Field of the Invention
The present invention relates to a method of thermally activating a heat-sensitive adhesive sheet with a heat-sensitive adhesive layer, and a thermal activation device therefor.
2. Description of the Related Art
Heat-sensitive adhesive sheets with a heat-sensitive adhesive layer that develops adhesion when heated, as those disclosed in JP 11-79152 A and JP 2003-316265 A, have been in practical use for some time now. Such heat-sensitive adhesive sheets have advantages including being easy to handle since the sheets are not adhesive prior to heating and producing no factory wastes since they do not need release paper. A thermal head, which is usually employed as a printing head in a thermal printer, is sometimes used to heat this type of heat-sensitive adhesive sheet and to thereby make its heat-sensitive adhesive layer develop adhesion. This is advantageous particularly when a heat-sensitive adhesive sheet is printable on one side, for thermal heads similar in structure can be used for printing and thermal activation. In a common thermal head, plural heating elements which can be driven separately from one another are arranged into an array.
A heat-sensitive adhesive sheet is given full adhesion by, in general, driving all heating elements which face a heat-sensitive adhesive layer of the sheet while the entire surface of the sheet is passed over the thermal head, in other words, by heating throughout the entire surface of the heat-sensitive adhesive layer. Usually, a standard driving energy to obtain desired heat generation characteristics through normal driving of one heating element is determined in advance, and each heating element receives the standard driving energy when the thermal head is driven.
In the case where a heat-sensitive adhesive sheet is required to have adhesion strong enough to prevent the sheet from peeling easily, the standard driving energy is supplied to every heating element facing a heat-sensitive adhesive layer of the sheet. On the other hand, in the case where a heat-sensitive adhesive sheet is required to have weak adhesion that allows a user to peel off the sheet by hand, the overall adhesion of the heat-sensitive adhesive sheet can be made weak by creating density data for activation and activating the sheet in accordance with the density data as disclosed in JP 2001-48139 A. A desired level of adhesion thus can be obtained by adjusting the density of a region to be activated.
As described, prior art gives a heat-sensitive adhesive sheet strong adhesion by directly heating and thermally activating the entire surface of a heat-sensitive adhesive layer of the heat-sensitive adhesive sheet by an opposing heating element. A drawback thereof is great power consumption in the thermal activation process. For instance, when a thermal activation device having a thermal head is driven by battery power, the battery will be spent in a short period of time from the thermal activation process.
Another drawback is large electric current consumption resulting from driving every heating element with the standard driving energy, which represents the amount of energy used to obtain desired heat generation characteristics through normal driving of one heating element. This means that a power source of large capacity is necessary in order to increase the speed of thermal activation and shorten the time it takes to thermally activate the entire surface of the heat-sensitive adhesive layer, and a large-capacity power source is large in size, weight and cost. If a power source of relatively small capacity is employed to reduce electric current consumption, thermal activation slows down, prolonging the time to finish thermally activating the entire surface of the heat-sensitive adhesive layer and lowering the work efficiency.
Still another drawback is that a large amount of heat is accumulated because all the heating elements facing the heat-sensitive adhesive layer are driven and generate heat until the entire surface of the heat-sensitive adhesive sheet finishes passing the thermal head. The large heat accumulation raises the temperature of the thermal head greatly and, for the purpose of protecting the thermal head, continuous use of the thermal head is limited to a short period of time. When the temperature of the thermal head reaches, for example, 80° C. or higher, the thermal activation device has to be shut down to avoid damage and transformation from heat.
The conventional thermal activation method thus has drawbacks of large power consumption, electric current consumption, and heat accumulation.
The invention disclosed by JP 2001-48139 A is capable of reducing power consumption, electric current consumption, and heat accumulation since it provides in a heat-sensitive adhesive layer a region that is not thermally activated, but this structure has been proposed in the first place to weaken the adhesion of the layer. Prior art has never produced a thermal activation device that makes a heat-sensitive adhesive sheet develop strong adhesion while cutting power consumption, electric current consumption, and heat accumulation.